Project Summary/Abstract B1 cells represent a distinct lymphocyte lineage and a fundamental component of the immune system. B1 cells are associated with autoimmunity and malignancy, and are responsible for the production of natural immunoglobulin that fulfills a critical anti-bacterial function. PD-L2 is a B7 family member previously reported to be expressed primarily by macrophages and dendritic cells but now shown by work from this laboratory to be expressed by B1 cells. Not only do B1 cells expess PD-L2, but PD-L2 expression marks a subset of B1 cells that is much more repertoire skewed, that results from increased proliferation, and that is particularly efficient at antigen presentation, in comparison to PD-L2 nonexpressing B1 cells and to B2 cells. The long term objective of this proposal, and of previous projects in this sequence, is to understand how B1 cells get to be the way they are, and what is the role and function of B1 cells within the immune system. PD-L2 expression by a subset of B1 cells provides a new and valuable tool to address these issues. It[unreadable]s value lies in the inability of other lymphocytes to express PD-L2, implying that the regulation of PD-L2 expression in B1 cells reflects a fundamental transcriptional feature of this lineage. It[unreadable]s value further lies in the lateralization of characteristics normally associated with the B1 cell lineage to a smaller subset of B1 cells, implying a new paradigm wherein PD-L2 plays a role in producing those characteristics or is produced by them. The specific aims of this proposal are to: 1) Determine the regulatory elements that control PD-L2 expression in B1 cells through an analysis of promoter sequences and binding factors; 2) Determine the developmental progression of PD-L2-expressing B1 cells through the study of phenotypically defined early stages in B cell differentiation; and, 3) Determine the role of PD-L2 in specifying the unique features of PD-L2-expressing B1 cells through manipulation of PD-L2 expression in mature and developing B1 cells. The results of this study are expected to provide new information about unique features of gene regulation in B1 cells and about the mechanism by which PD-L2 expression correlates with several characteristics previously attributed to B1 cells in general. These are fundamental issues regarding B1 cell behavior, through the study of which much will be learned regarding the place and activity of B1 cells within the immune system. Elucidation of these points is likely to provide new targets and strategies to ameliorate the progression of autoimmune dyscrasias, to influence the course of malignant diseases, and to assist in enhancing immunity in normal and immune-deficient patients.